Access to devices is prohibited by default for the sake of your privacy. You must clearly give permission to use of a camera and/or microphone.
When a site requests access to your device, you may see a pop-up window at the top of the browser, similar to this:
By clicking on the "Allow" button, the sound from your microphone or video from the webcam becomes available to the site only at the moment his use.
If you click "Block", the site will not be able to access your devices, but you will not be able to continue using its corresponding functions.
In order to return access to devices or, conversely, deny it, you can open the appropriate site settings in your browser.
Different browsers have these settings in different places, but most often there is a button somewhere near the address bar as in the image above.
Access to devices may also be denied to the browser itself at the level of your operating system settings. To grant access, you need to open its privacy settings and make the appropriate changes, for example as described here for Windows OS.
The website webcammictest.com does not store your data on its servers without your consent, but records and plays them only in your browser window.